1. Field of the Invention
The present invention relates to an apparatus and a method for measuring a signal from a femto base station in a mobile communication system. More particularly, the present invention relates to an apparatus and a method for measuring a signal from a femto base station of a user equipment registered to the femto base station in a mobile communication system.
2. Background of the Invention
In a cellular wireless communication system, a geographical condition in a cell, a distance between a user equipment and a base station, or movement of the user equipment may deteriorate a channel condition and block the communication between the user equipment and the base station. For example, even in the service coverage area of the base station, a sealed structure such as office or house forms a shadow area. In a case where the user equipment travels in the shadow area, the base station may not smoothly communicate with the user equipment because of the poor channel condition between the base station and the user equipment.
To address this service problem in the shadow area and to offer a high-rate data service, the wireless communication system may provide a femtocell service. The term “femtocell” denotes a small cell coverage area established by a compact base station which accesses a mobile communication core network via a broadband network. Typically, the compact base station is installed indoors, such as in an office or a house. The compact base station, which is a low-power output base station installed by a user, may be referred to as a micro base station, a self-configurable base station, an indoor base station, a home base station, or a femto base station. Hereinafter, the compact base station is referred to as the femto base station. The femto base station may distribute radio resources between the user equipment and a macro base station to the femtocells so that the user equipment may reliably send and receive high-speed data with low cost and the macro base station may distribute the excessive radio resources focused on a particular user equipment, through the femto base station.
The femto base station works with the macro base station, and the user equipment should be serviced so as to be able to be handed over between the femto base station and the macro base station. That is, the user equipment should be provided with an improved service that allows for a handing over between the macro base station and the femto base station according to the radio condition.
The service coverage area corresponding to one macrocell may include a large number of service coverage areas corresponding the femtocells, such as thousands of femtocells. A Physical Cell ID (PCID), which is one ID used for identifying the femtocells, may be duplicated within the macrocell. Hence, it is difficult for the macrocell to identify the overlapping femtocells merely with the PCID of a neighbor cell which is reported to the macrocell by the user equipment using a measurement report message. This is the important consideration when only the user equipment that is allowed to access the femtocell is handed over to the corresponding femtocell. Herein, the femtocells are divided into cells allowing the access of every user equipment and cells (hereafter, referred to as Closed Subscriber Group (CSG) cells) allowing the access of only the allowed user equipment.
To address this problem, mobile communication standardization groups such as 3rd Generation Partnership Project (3GPP) are discussing an approach for supporting the mobility of the user equipment merely with the PCID and an approach for supporting the mobility of the user equipment with a Global Cell ID (GCID) which is a unique cell ID for identifying the femtocell, together with the PCID. However, the approach for supporting the mobility solely with the PCID faces realistic difficulties. The approach for supporting the mobility using the GCID together with the PCID is under discussion.
According to a conventional method, the user equipment reports the GCID together with the PCID to the macrocell, and the macrocell supports the mobility of the user equipment using those IDs. The conventional method is explained in more detail below.
First, the user equipment receives a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS) from its neighbor cell and thus obtains the PCID of the corresponding neighbor cell. However, since it is hard to identify the femtocell merely with the PCID, the user equipment receives a Master Information Block (MIB) and a System Information Block (SIB) from the neighbor cell and thereby obtains a General Cell ID (GCID) which is the unique cell ID of the corresponding neighbor cell. Next, the user equipment sends a measurement report message including the obtained PCID and GCID of the neighbor cells, to its serving macro base station. Based on the measurement report message, the serving macro base station determines whether the user equipment may access the corresponding neighbor cell. According to the determination, the serving macro base station may command the user equipment to hand over to the corresponding neighbor cell. Alternatively, the user equipment determines whether the corresponding neighbor cell is the accessible CSG cell by comparing a list of its accessible CSG cells (i.e., the allowed CSG list), which is managed by itself, with the obtained GCID. When determining the access, the user equipment may send the measurement report message including the obtained PCID and GCID of the neighbor cell, to the serving macro base station. Based on the measurement report message, the serving macro base station may command the user equipment to hand over to the corresponding neighbor cell.
As such, as the PCID is redundant and the user equipment needs to obtain the GCID of the neighbor cell, the measurement time to acquire the PCID and the GCID is lengthened and thus the performance of the user equipment is degraded. In particular, when there are a great number of femtocells in the area, the user equipment iteratively receives the MIB and the SIB of each femtocell. As a result, the performance degradation of the user equipment may be aggravated.
In addition, as the user equipment or the macro base station determines whether the neighbor cell is the accessible femtocell of the user equipment, the performance of the user equipment is degraded. In particular, when there are a great number of femtocells in the area, the user equipment or the macro base station needs to continually determine whether the neighbor cell is an accessible femtocell of the user equipment. Provided that the time of about 20 ms is required for each neighbor cell, the performance degradation of the user equipment may be worsened.